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Analyzing a Product's Working Environment To Aid Part Designs

Design engineers and OEMs can choose from a vast combination of materials and processes to create parts and components for new or existing products. The dizzying array of options can include plastics; die-cast metals; such metals as iron, steel and copper-base alloys; and at the most costly end of the spectrum, specially alloyed or heat-treated ferrous alloys.

Obviously, no one material is the best choice for all applications. So how to decide? To help designers make the best selection, the Diecasting Development Council (DDC) has created a systematic plan that focuses on the product's actual working environment.

Based on the experience of hundreds of designers and die casters, the DDC recommends that designers carefully evaluate six aspects of every product's working environment before settling on a production method.

Those six critical factors are:

Operating temperature.

Applied loads.

Interfacing components and subassemblies.

Electrical and electronic requirements.

Corrosion.

Accidental or unexpected conditions.

Operating Temperature

Like so many things, temperature is not always what it seems to be. Several key questions merit consideration.

Cyclic versus steady state: In a cyclic environment, the maximum temperature often doesn't represent what temperature the part will actually reach. For example, in gasoline engines, die-cast aluminum and magnesium pistons are often exposed to temperatures above 3,000°F (1,648°C). However, the piston is exposed to this extreme for only a brief portion of the cycle. Gas-turbine wheels, on the other hand, operate in an environment of about 2,200°F (1,204°C), yet require high-temperature alloys. Why? Because the turbine wheels are exposed to a continuous flow of hot gases - a steady-state environment.

Continuous or transient: Transient high temperatures, or "spikes," often occur in heat-generating devices, such as furnaces or internal-combustion engines, especially when the device is shut down. The temperature can rise briefly while heat dissipation begins through convection. However, these spikes are almost always brief and die-cast components are often designed to easily weather these extremes. Eliminating the potential incorporation of die-cast components simply because of these transient temperature spikes can also wrongly eliminate a viable cost-saving manufacturing process from an OEM design.

Internal and external: The temperature surrounding a component is frequently much different than the level reached by the component itself. For example, the temperature under a car's hood often reaches 220°F (104°C). Yet fuel pumps, which are frequently die cast, are also exposed to a steady stream of lower temperature fuel. The result: the pump itself runs at a temperature somewhere between the two - substantially less than the "underhood" air temperature.

Applied Loads

Applied loads are always a concern for equipment makers. After all, these loads cause stress and deflections and need to be vigorously assessed.

Designers should consider four types of loads when designing:

Long-term or continuous loads, with their ability to induce creep or stress-corrosion cracking.

Short-term loads that are applied relatively few times.

Cyclic loads that are repeated thousands or millions of times in a product's life, which may induce fatigue failures.

Rarely applied impact loads that, while unusual, still have the potential to cause gross distortion or component fracture.

Many of today's engineered die-casting alloys can withstand the four load types described above. They are virtually always superior to their plastic equivalents, and often match up well to more-costly copper-base and ferrous-alloy options. Thorough data on die-cast alloy strength, rigidity, and endurance provide ample information to help designers choose the best materials.

Interfacing Components and Subassemblies

Countless OEMs have lowered product costs, increased manufacturing flexibility, and improved product performance and life expectancy by replacing complex assemblies with well-designed die-cast parts. Often, in fact, die castings are integrated with formerly attached components, saving time and dramatically reducing the total number of parts required. Creating components from fewer or even single parts provides an elegant solution to the handling and assembly of multicomponent parts.Designers should consider three characteristics of interfacing components and assemblies when searching for product improvements:

Dissimilar materials: Examples include materials that might have conflicting coefficients of thermal expansion and the potential for galvanic corrosion.

Differences in material properties: For example, creep properties may prohibit or permit the use of tapped threads or press-fit inserts. Likewise, ductility (or the lack of it) may limit options when considering crimping, staking and swaging.

Mounting surfaces: More or less dimensional precision may be required depending on the stress response in both the casting and the mounting component.

Electrical and Electronic Requirements

For makers of electronic devices, electromagnetic interference (EMI) is a constant concern. The problem has resulted in the adoption of military and ASTM standards, as well as action from the Federal Communications Commission (FCC). EMI shielding is not only a concern for devices generating EMI but also to ensure operating integrity of components from external sources.

One of the best ways to achieve EMI shielding is to enclose the device in question within a conductive cover. When EMI impinges on a conductive cover, ohmic currents are induced in it and dissipated as heat. Designing plastic components for these types of applications become very costly, because of the need to add shielding through foil or mix metal particles in the plastic itself. On the other hand, the inherent electrical conductivity of die-casting alloys usually meets or exceeds EMI standards and provides added impact strength benefits.

Corrosion

Designers must consider two types of corrosion when designing components. One is atmospheric corrosion, caused by water, pollutants, and other chemicals in the atmosphere. The other is galvanic corrosion, resulting from the electrochemical reaction that occurs at the interface of dissimilar metals exposed to liquids capable of conducting electricity.

Atmospheric corrosion is typically not an issue for die-cast parts. In fact, the initial process usually forms a protective coating the castings, effectively blocking progressive damage.

The possibility of galvanic corrosion should be carefully considered, though. The popular die-casting family of alloys - aluminum, magnesium and zinc - are all "active" metals and potentially subject to problems.

The die-casting industry is familiar with corrosion issues and has developed a number of low-cost solutions. These include selecting alloys that are galvanically compatible, designing out features that trap and retain moisture, and providing moistureproof barriers at the casting interface.

Accidental or Unexpected Conditions

The product engineer must try and foresee as many unusual conditions as possible for each component. Some conditions, such as stone (impact) damage and environmental (acid-rain) pollution simply require a margin of safety established and designs created with these potential conditions in mind and the reasonable levels of protection factored in.

Here is where the understanding of the die-casting process along with the complete review of the large selection of alloys is a key step in preliminary new part design. When it comes to standing up to the unexpected, the superior strength of die castings often do provide protection - even against the unknown.

Conclusion

In conclusion, it's obvious that there is no perfect material or production process for every part and component. Knowing what working environment the final product will be in remains a key factor. But once this is established, the relatively low cost and superior performance of parts produced using the advanced die-casting process, along with a litany of alloy choices, with various physical and mechanical properties, offer OEM part designers competitive solutions to their design application and material concerns.

This article is adapted from "Product Design for Die Casting," published by the North American Die Casting Association. The Diecasting Development Council provides a wealth of technical literature and design assistance to design engineers and OEMs. For more information, write to the DDC at 9701 W. Higgins Rd., Suite 855, Rosemont, IL, 60018-4721, phone: (847) 292-3625, fax: (847) 292-3613, e-mail: ddc@diecasting.org. Visit the DDC Web site at http://www.diecasting.org/DDC.